1. Field of the Invention
The present invention relates to an aluminum alloy plate excelling in filiform corrosion resistance. More particularly, the present invention relates to a bake-hardenable Alxe2x80x94Mgxe2x80x94Sixe2x80x94Cu aluminum alloy plate, excelling in filiform corrosion resistance, which is suitably used as a material for transportation devices such as an automotive outer body panel, and to a method of fabricating the same.
2. Description of Background Art
In recent years, reduction of the weight of automobiles has been demanded in order to improve fuel consumption from the viewpoint of environmental protection, etc. To deal with this demand, an aluminum alloy plate has been partly used for an automotive outer body panel, in place of a cold-rolled steel plate has conventionally been used.
As examples of aluminum alloys currently used for an automotive outer body panel in practice, Alxe2x80x94Mg alloys such as A5022, A5023, and A5182, and Alxe2x80x94Mgxe2x80x94Si alloys such as A6111, A6016, and A6022 can be given. Alxe2x80x94Mg alloys excel in formability. However, Alxe2x80x94Mg alloys do not exhibit bake-hardenability, thereby exhibiting inferior dent resistance.
Since Alxe2x80x94Mgxe2x80x94Si alloys exhibit excellent bake-hardenability, Alxe2x80x94Mgxe2x80x94Si alloys exhibits superior dent resistance. However, formability of Alxe2x80x94Mgxe2x80x94Si alloys is insufficient. The addition of Cu provides improved formability to Alxe2x80x94Mgxe2x80x94Si alloys due to an increased r value (Lankford value). However, addition of Cu tends to cause intergranular corrosion to occur, thereby resulting in decrease in corrosion resistance, in particular, filiform corrosion resistance. Therefore, the Cu content in the A6016 alloy and the A6022 alloy is limited to 0.20% or less and 0.11% or less, respectively. In addition, A6111 alloys which contain 0.50-0.9% of Cu may exhibit inferior corrosion resistance.
Japanese Patent Application Laid-open No. 10-176233 proposes a method for preventing intergranular corrosion by adding Zn to an Alxe2x80x94Mgxe2x80x94Si alloy containing Cu. The addition of Zn decreases the electric potential of an electrochemical matrix, thereby decreasing the potential difference between Mg2Si and the matrix. This prevents Mg2Si precipitated on the grain boundaries from being dissolved under corrosive environment, whereby intergranular corrosion can be prevented. In this method, the Cu content is also limited to 0.8% or less. If the Cu content exceeds 0.8%, corrosion resistance decreases.
Japanese Patent Application Laid-open No. 10-237576 proposes an Alxe2x80x94Mgxe2x80x94Si alloy containing 0.25-1.0% of Cu and exhibiting improved corrosion resistance which is used for an automotive outer body panel, wherein Pb, As, Sn, and other impurity concentrations in a Zn substrate plating layer used for zinc phosphate treatment and paint treatment are limited. However, the object of this technique is to improve corrosion resistance of the material by use of the substrate plating layer for conversion treatment, but not to improve corrosion resistance of the Alxe2x80x94Mgxe2x80x94Sixe2x80x94Cu alloy itself. This method involves difficulty in managing the plating solution.
The present inventors have conducted extensive experiments and studies to solve the above problems relating to an Alxe2x80x94Mgxe2x80x94Sixe2x80x94Cu alloy used for an automotive outer body panel by improving corrosion resistance of the alloy, and to produce an Alxe2x80x94Mgxe2x80x94Sixe2x80x94Cu alloy exhibiting excellent formability, excellent intergranular corrosion resistance, and improved filiform corrosion resistance after painting. The present inventors have conducted studies from the viewpoint of clarifying the relation between intergranular/filiform corrosion resistance and intermetallic compounds precipitated inside or boundaries of the crystal grains during the fabrication process. Accordingly, an object of the present invention is to provide an Alxe2x80x94Mgxe2x80x94Sixe2x80x94Cu alloy suitable for an automotive outer body panel which excels in strength and formability and exhibits improved filiform corrosion resistance, and a method of fabricating the same.
An aluminum alloy plate excelling in filiform corrosion resistance and a method of fabricating the same according to the present invention are characterized as follows.
(1) An aluminum alloy plate excelling in filiform corrosion resistance, comprising 0.25-0.6% of Mg (mass %, hereinafter the same), 0.9-1.1% of Si, 0.6-1.0% of Cu, and at least one of 0.20% or less of Mn and 0.10% or less of Cr, with the balance consisting of Al and impurities, wherein the number of Q phases (Cuxe2x80x94Mgxe2x80x94Sixe2x80x94Al phases) with a size of 2 xcexcm or more in diameter present in a matrix is 150 per mm2 or more.
(2) A method of fabricating an aluminum alloy plate excelling in filiform corrosion resistance, comprising: homogenizing an ingot of an aluminum alloy which comprises 0.25-0.6% of Mg, 0.9-1.1% of Si, 0.6-1.0% of Cu, and at least one of 0.20% or less of Mn and 0.10% or less of Cr, with the balance consisting of Al and impurities, at a temperature of 530xc2x0 C. or more; cooling the ingot to 450xc2x0 C. or less at a cooling rate of 30xc2x0 C./hour or less; hot-rolling the ingot; cold-rolling the hot-rolled product; and providing the cold-rolled product with a solution heat treatment.
(3) A method of fabricating an aluminum alloy plate excelling in filiform corrosion resistance, comprising: homogenizing an ingot of an aluminum alloy which comprises 0.25-0.6% of Mg, 0.9-1.1% of Si, 0.6-1.0% of Cu, and at least one of 0.20% or less of Mn and 0.10% or less of Cr, with the balance consisting of Al and impurities, at a temperature of 530xc2x0 C. or more; cooling the ingot to room temperature; heating the ingot to 500xc2x0 C. or more and allowing the ingot to stand for 30 minutes or more; cooling the ingot to 450xc2x0 C. or less at a cooling rate of 30xc2x0 C./hour or less; hot-rolling the ingot; cold-rolling the hot-rolled product; and providing the cold-rolled product with a solution heat treatment.
(4) In the method of fabricating an aluminum alloy plate excelling in filiform corrosion resistance described in the above (2) or (3), the solution heat treatment may be carried out at 550xc2x0 C. or less for 30 seconds or less.
The effects of alloy components of the Alxe2x80x94Mgxe2x80x94Sixe2x80x94Cu alloy plate of the present invention and reasons for the limitations thereof are described below. Mg bonds to Si to form intermetallic compounds (Mg2Si), thereby improving the strength of the alloy. The Mg content is preferably 0.25-0.6%. If the Mg content is less than 0.25%, the effect may be insufficient. If the Mg content exceeds 0.6%, bendability may decrease. The Mg content is still more preferably 0.30-0.55%.
Si forms an intermetallic compound (Mg2Si) in the presence of Mg, thereby improving the strength of the alloy. The Si content is preferably 0.9-1.1%. If the Si content is less than 0.9%, the effect of the strength improvement may be insufficient. If the Si content exceeds 1.1%, bendability may decrease.
Cu improves the strength and formability. The Cu content is preferably 0.6-1.0%. If the Cu content is less than 0.6%, formability may be insufficient. If the Cu content exceeds 1.0%, corrosion resistance may decrease.
Mn and Cr miniaturize the crystal grain. The Mn content and the Cr content are preferably 0.20% or less and 0.10% or less, respectively. If the Mn content and the Cr content respectively exceed the upper limits, elongation may decrease. This results in a decrease in bendability and formability. The Mn content and the Cr content are still more preferably less than 0.10% and 0.07% or less, respectively.
Note that the effects of the present invention are not impaired if elements generally included in an Alxe2x80x94Mgxe2x80x94Sixe2x80x94Cu alloy, such as 0.2% or less of Ti, 0.1% or less of B, 1.0% or less of Fe, 0.5% or less of Zn, and 0.05% or less of Zr, are present in the Alxe2x80x94Mgxe2x80x94Sixe2x80x94Cu alloy.
In a matrix of the alloy plate of the present invention having the above composition, it is important that 150 per mm2 or more of Q phases (Cuxe2x80x94Mgxe2x80x94Sixe2x80x94Al phases) with a size of 2 xcexcm or more in diameter are present. This precipitation configuration provides preferable corrosion resistance.
Corrosion configuration of the Alxe2x80x94Mgxe2x80x94Sixe2x80x94Cu alloy is mainly intergranular corrosion. Filiform corrosion of the painted plate is considered to be caused by intergranular corrosion of the material below the paint. Therefore, it is necessary to improve intergranular corrosion resistance of the material to prevent filiform corrosion of the painted plate. Intergranular corrosion resistance of the material depends on the configuration of precipitates present on the crystal grain boundaries of the material matrix or precipitate free zones (PFZs).
The Q phases (Cuxe2x80x94Mgxe2x80x94Sixe2x80x94Al phases) are crystallized during casting or precipitated in a step during the fabrication of the alloy plate. The Q phases contain undissolved Cu and are mainly crystallized or precipitated in the crystal grains. When the Q phases are dissolved during high-temperature heat treatment such as a solution heat treatment, the amount of solid-solution of Mg, Si, and Cu increases. This tends to cause Mg2Si compounds containing Cu to be precipitated on the crystal grain boundaries, whereby the potential difference among the precipitates, PFZs, and inside the grains increases. This decreases intergranular corrosion resistance, whereby filiform corrosion tends to occur in the resulting painted plate.
In the present invention, the number of Q phases (Cuxe2x80x94Mgxe2x80x94Sixe2x80x94Al phases) with a size of 2 xcexcm or more in diameter is preferably 150 per mm2 or more. If the number is less than 150 per mm2, filiform corrosion tends to occur due to decreased corrosion resistance. The number of Q phases can be determined by Electron Probe Micro Analyzer (EPMA). The number of Q phases is determined by counting the number of spots with a size of 2 xcexcm or more in diameter where Mg, Si, and Cu are simultaneously present.
The method of fabricating the aluminum alloy plate of the present invention is described below. In the present invention, an aluminum alloy having the above composition is cast by Direct Chill (DC) Casting Process. The resulting ingot is subjected to homogenization, hot-rolling, and cold-rolling to prepare a plate material. The plate material is then provided with a solution heat treatment to obtain a T4 temper material.
Conventionally, it is considered to be preferable for the homogenization temperature of Alxe2x80x94Mgxe2x80x94Si alloys to be as high as possible to promote solid-solution of Mg and Si and cutting/spheroidizing of Alxe2x80x94Fexe2x80x94Si constituent particles. However, high-temperature homogenization causes dissolution of the Q phases crystallized during casting, thereby decreasing filiform corrosion resistance of the resulting painted plate. This requires precipitating of Q phases after homogenization.
As a result of various tests and examinations, the present inventors have confirmed that the Q phases are reprecipitated by cooling the homogenized ingot at a rate as slow as possible. The present inventors have conducted further examinations and found the optimum homogenization conditions. Specifically, the homogenization temperature is preferably 530xc2x0 C. or more. If the homogenization temperature is less than 530xc2x0 C., the amount of solid-solution of Mg and Si decreases, whereby the strength may become insufficient. Moreover, bake-hardenability decreases. The homogenization temperature is still more preferably 560xc2x0 C. or more.
In the case of hot-rolling the ingot immediately after homogenization, the homogenized ingot is cooled to 450xc2x0 C. or less at a cooling rate of 30xc2x0 C./hour or less, thereby causing the Q phases to be reprecipitated during cooling. Hot-rolling is started at this temperature. If the cooling rate exceeds 30xc2x0 C./hour or the hot-rolling starting temperature exceeds 450xc2x0 C., precipitation of the Q phases becomes insufficient whereby filiform corrosion resistance decreases. The hot-rolling starting temperature is still more preferably 420xc2x0 C. or less. In this case, the homogenized ingot is cooled to 420xc2x0 C. or less at a cooling rate of 30xc2x0 C./hour or less. Hot-rolling is started at this temperature.
In the case of cooling the homogenized ingot to room temperature and heating the ingot before hot-rolling, the homogenized ingot is cooled to room temperature. The ingot is then heated to 500xc2x0 C. or more and allowed to stand for 30 minutes or more. The ingot is then cooled to 450xc2x0 C. or less at a cooling rate of 30xc2x0 C./hour or less. Hot-rolling is started at this temperature. If the cooling rate exceeds 30xc2x0 C./hour or the hot-rolling starting temperature exceeds 450xc2x0 C., precipitation of the Q phases becomes insufficient, whereby filiform corrosion resistance decreases. The hot-rolling starting temperature is still more preferably 420xc2x0 C. or less. In this case, the ingot is heated to 500xc2x0 C. or more and cooled to 420xc2x0 C. or less at a cooling rate of 30xc2x0 C./hour or less. Hot-rolling is started at this temperature.
After hot-rolling, the hot-rolled product is optionally provided with intermediate annealing and then cold-rolled to obtain a plate material with a predetermined thickness. The plate material is then provided with solution heat treatment to obtain a T4 temper material. It is preferable to perform solution heat treatment at a low temperature for a short period of time to prevent decomposition of the Q phases. In an equilibrium state, a dissolution reaction shown by xe2x80x9cAl+Q phasexe2x86x92Liq.+Mg2Si+Sixe2x80x9d occurs at 529xc2x0 C. In a solution heat treatment by rapid heating, the Q phases are not completely dissolved at 550xc2x0 C. or less. Therefore, it is preferable to perform the solution treatment at 550xc2x0 C. or less. The solution treatment temperature is still more preferably less than 529xc2x0 C. The solution treatment is preferably performed for 30 seconds or less, and still more preferably for 10 seconds or less. Solution treatment by rapid heating using a continuous annealing line (CAL) is suitably employed.